Abstract: Drilling fluid formulation and density are key parameters of pollution and evalution in coal reservoir. Through the dissection of the CBM wells in Sihe mining area, the paper observed the pollution characteristics of the drilling solid-phase material and established the relationship between drilling fluid density and depth of solid-phase invasion. The results showed that for large fracture system of coal reservoir, especially exogenous joints system as migration pathways and storage space of solid-phase materials in drilling fluid, the range of coalbed methane well pollution was up to several meters. The higher the drilling fluid density, the thicker the mud cake in the coal reservoir fractures, the invesion depth of filtrate was reduced relatively. On this basis, the range of CBM drilling was predicted in Baiyanghe mining area, Xinjiang. Because of the broken coal texture of coal reservoir and adoption of along seam drilling technology in Xinjiang, the residence time of drilling fluid is long, the pollution of the solid-phase materials is relatively severe.